The present application relates generally to low-frequency continuous-time filters, and more specifically to integrators for low-frequency continuous-time filters that can be implemented in integrated circuits.
In recent years, there has been an increasing need for continuous-time filters that can be implemented in integrated circuits (ICs). By implementing functional circuit components such as filters in ICs, significant reductions in the size, cost, and complexity of a target system can be achieved. A conventional continuous-time filter implementable in an IC is the transconductor-capacitor (gm-C) filter, in which “gm” represents the transconductance of a transconductor and “C” represents the capacitance of a capacitor. The conventional gm-C filter includes one or more gm-C integrators. Each gm-C integrator typically operates in the current mode, i.e., both the input and output signals of the gm-C integrator take the form of currents. Such gm-C filters have been widely used in low-voltage high-frequency applications such as cellular telephones. For example, in cellular telephones, integrated gm-C filters may be employed to reconstruct received signals, and to perform anti-aliasing prior to signal transmission.
Not only is there a need for integratable continuous-time filters in high-frequency applications, but there is also a need for integratable continuous-time filters in low-frequency applications. Such low-frequency applications include biomedical systems that sense and process bioelectrical signals and that require filters having cutoff frequencies below 100 Hz; speech recognition systems and sound detecting/processing devices such as hearing aids that require filters operating in the audio frequency range; and, seismic and machine surveillance systems that require low-frequency filters for monitoring and for analyzing signals with frequencies typically ranging from about 0.1 Hz to 100 Hz. Low-frequency signal processing is also employed in some medium to high-frequency applications such as averaging filters in root mean square (RMS) detectors, automatic gain control (AGC) circuits, and phase locked loop (PLL) circuits.
However, using gm-C filters in low-frequency applications can be problematic, especially when implementing the filters in an IC. This is because in low-frequency applications, it is generally desirable to use gm-C filters that have a low transconductance. For a typical gm-C integrator, however, the transconductance of the transconductor is inversely proportional to the capacitance of the capacitor. As a result, as the transconductance of the gm-C integrator is reduced, the amount of area required to implement the capacitor is increased, thereby reducing the amount of area available for the gm-C filter and other circuits in the IC. Although the larger capacitance of the low-frequency gm-C integrator may be implemented using one or more components external to the IC, such use of external components typically increases the size, cost, and/or complexity of the target system.
It would therefore be desirable to have an integrator suitable for use in low-frequency continuous-time filters and implementable in an IC that avoids the shortcomings of the above-described conventional gm-C integrator.